According to the characteristics of driving elements of the currently available fan motors, when an input supply voltage thereof is high, the fan is driven to rotate at full speed to increase a cooling efficiency thereof, and when the input supply voltage thereof is low, the fan is driven to rotate at a lowest possible speed. That is, the fan speed is adjusted according to a linear change in the high and low level of the input supply voltage. In this manner, the lowest rotating speed of the fan at the low input supply voltage is a fixed value that could not be varied. Therefore, the fan has limited applications and could not always meet consumers' needs.
It is therefore tried by the inventor to develop an improved motor driving system to eliminate drawbacks existed in the conventional fan motors.